This invention relates to the measurement of acoustic wave travel time in a fluid medium, with particular application to acoustic pyrometry.
Techniques are known for measuring the transit time of acoustic waves from a transmitting location to a receiving location through a fluid medium. Systems using both pulsed waves and continuous waves have been proposed and used in the past for various purposes. In pulsed systems, the transit time is typically measured by noting the time difference between the generation of an acoustic pulse at the transmitting location and the receipt of the same acoustic pulse at the receiving location. In continuous wave systems, the phase difference between the continuous wave at the transmitting location and at the receiving location provides an indirect measurement of the transit time. The transit time thus obtained is typically used to compute the velocity of the acoustic waves in the medium. In acoustic pyrometry, the computed velocity is used to compute the temperature of the fluid using a well-known relationship between acoustic velocity and temperature. For a fuller discussion of the pulsed technique see M. W. Dadd, "Acoustic Thermometry In Gases Using Pulse Techniques", High Temperature Technology, Vol. 1, No. 6, November, 1983. For a fuller discussion of the continuous wave technique see U.S. Pat. No. 4,215,582.
While both the pulsed and continuous wave techniques have been found to be useful in many applications, each is demonstrably unsuitable in extremely noisy environments in which erroneous transit time determinations occur due to the masking presence of substantial noise signals and multiple transmission paths for the acoustic wave. One example of such a noisy environment is in the field of industrial boilers, such as modern utility boilers, chemical recovery boilers and refuse boilers. Added to the noise problem is the compounding adverse effect of attenuation of acoustic waves due to scattering of the waves by temperature and velocity gradients (the latter in a moving fluid), and the masking effect of acoustic waves arriving at the receiving location via reflected boundary paths. While many efforts have been made to improve the reliability of acoustic transit time measurement in noisy environments, such efforts have not met with success to date.